The present invention relates to directional microphone assemblies, and particularly to those which may be used in applications which benefit from minimum visual intrusion. A primary example of these applications is use in vehicle cabins for speech pickup for hands-free telephony and other communication and control applications. Both omnidirectional and directional microphones have been used for this purpose. These are generally mounted on interior surfaces, most typically at a forward, central headliner position or near the top of the driver side roof-support pillar. Omnidirectional microphones have also been mounted behind such surfaces, with sound entering through a relatively small surface hole or group of holes or slots. This behind-the-surface mounting is aesthetically preferable to over-the-surface mounting and eliminates the need for designers to consider microphone styling and color.
Directional microphones can produce significant performance advantages over omnidirectional ones in the vehicle environment, however, and are therefore preferable. Compared to an omnidirectional microphone, an optimally positioned, well-designed, surface-mounted first-order directional microphone can produce a several decibel advantage in the ratio of speech pickup to general road noise, and an even greater advantage in rejection of localized ventilation noises and return telephony audio.
Although encased directional microphones, where the microphone elements are contained within mostly acoustically opaque housings, are presently available for other applications, most notably for use in hearing aids and, more recently, in some portable telephones and computer monitors, these prior art approaches have requirements and characteristics which make them less than optimum for subsurface applications such as the just described vehicle use. A typical prior art approach is shown cutaway in FIG. 1. A small (approximately 1 cm tall) electret microphone element 11 is mounted perpendicularly behind a thin surface 13. The front of the element diaphragm acoustically couples through tube 15 and surface hole 17 to the acoustic pickup region 19. Similarly, the rear of the element diaphragm acoustically couples through tube 21 and surface hole 23 to pickup region 19. Acoustic resistor 25 in tube 21, in conjunction with the enclosed rear volume 27 behind the element diaphragm, form a low-pass filter/delay for sound entering hole 23. This delay, in conjunction with the tube dimensions of the front and rear sound entry paths and the spacing distance between entry holes 17 and 23, forms a first-order directional pickup pattern in the pickup region which is directed along a line from rear entry hole 23 to front entry hole 17.
This prior art approach can be implemented to operate effectively over a useful frequency range. Its acoustical characteristics are, however, critically dependent on the individual and relative acoustical characteristics of the front and rear sound entry paths. Included in these paths are the mounting surface 13, surface holes 17 and 23, and anything which may be placed in front of them. Were such an assembly to be installed behind an automotive interior surface, the sound entry paths would be modified by considerable additional surface thickness with varying additional entry hole sizes and possibly by acoustically semi-transparent decorative covering material. These additional acoustical elements would degrade each of the front and rear sound entry paths differently, since each presents different acoustic impedance at entry holes 17 and 23. The driving force on the element diaphragm is derived from the difference of the pressures on its front and rear sides and may have a magnitude which is only a relatively small percentage of the individual front and rear pressure magnitudes. Relatively small unbalanced changes in the front and rear pressures can, then, result in much larger relative changes in the net diaphragm driving force, causing the mounted microphone assembly pickup characteristics to suffer severe degradation.
What is needed, then, is another approach to creating a subsurface directional microphone. It should be capable of attachment behind an interior surface, with acoustic entry provided by relatively small and unobtrusive openings. It should exhibit a high degree of insensitivity to the characteristics of the acoustic entry paths through the surface.
Therefore, an object of the present invention is to provide a directional microphone assembly which can be unobtrusively mounted behind a surface.
Another object is to provide such an assembly which exhibits greatly reduced sensitivity to variations in the acoustical coupling through the surface.
Another object is to provide such an assembly with reduced sensitivity to variations in microphone element characteristics.
Another object is to provide such an assembly with reduced sensitivity to very low frequency inputs.
Another object is to provide such an assembly with extended high frequency response.
A further object is to provide such an assembly which also includes an additional output with more extended low frequency response and reduced directionality.
Yet another object is to provide a similar assembly which can provide two or more directional patterns aimed in different directions.